If you need to grab a single frame from a video and turn it into a still image or add it to your Photos library, you can do it using a few steps even though a direct function for that doesn't exist in the current Photos app. Here are two methods you can use.
The Mac Photos app has some deep functionality and even regular users may not know some of these tips. Learn how to search by photo contents, convert Live Photos to long exposure pictures, export contact sheet PDFs, adjust and filter videos, create multiple libraries and much more.
You can share photos online using the Photos app on your Mac and iCloud. Others can view it on a public website on in their Photos app. You can also make the album private and allow others to add photos and comments to the album.
Your Photos app on your Mac will occasionally suggest groups of photos to view as Memories. You can view the photos in the Memory, or watch it as a slideshow. You can also save Memories and let Photos know if you'd like to see fewer memories like the one you are viewing.
MacOS Big Sur adds the ability for you to adjust, filter, crop, rotate and trim videos right in the Photos app, without needing to go to iMovie or Final Cut Pro. You can use the same editing techniques you use for photos, but applied to an entire video.
You can crop a photo using the tools that come with every Mac. If the photo is in the Photos app, you can use the cropping function in the editing tools there. If the photo is an independent file, you can use Preview or Markup Tools to crop it.
You don't need to launch Affinity Photo to use some of its tools. You can access basic adjustments, haze removal, retouch tools and other things right in the Mac Photos app.
You can use Affinity Photo to make adjustments and apply filters to your photos directly from the Photos app. By using adjustment layers, you can test out different changes and play with settings to get the result you want.
If you need to open PDF files or just want a decent image editing app that doesn't cost too much or require a subscription, there are plenty of options in the Mac App Store and elsewhere. All of these and 64-bit apps that work with macOS Catalina and beyond. See https://macmost.com/mac-photoshop-alternatives.html for links.
It seems that uploading to Instagram is something you can only really do from your phone. But by tricking the site into thinking your Mac is a phone, you can easily upload photos and viddos to Instagram in Safari on your Mac.
If you want to combine photos into a collage, you can do it using Pages. By using Page Layout mode with custom page dimensions, rulers, guides, box shapes and alignment tools, you can create a grid of photos that at any size. Then you can drag and drop pictures from the Photos app into each box. You can also use borders or even build creative designs with various shapes and sizes.
THe Photos app includes tools like albums and tags to help you organize your photo library. But if you like, you can skip that and just use built-in features that automatically group your photos by dates, locations, people or objects.
The Photos app on your Mac has a set of adjustment and editing tools. Get familiar with how these work by experimenting with them.
You can use the Mac Photos app to highlight one color in a photo and desaturate the rest. You can also do different things with different colors. A third-party app like Pixelmator Pro can be used as an external editor to saturate a color in only part of an image.
You can quickly and easily made a slideshow in the Photos app, but those can't be exported to share. By using the free iMovie app from Apple, you can build a slideshow from a set of photos in minutes and then export it as video to share. You can also customize each photo with filters, cropping, movement and text. You can add music, audio and narration to the slideshow too. The basic idea works on the iPad as well.
Sometimes you need to shrink an image in order to upload it to a web site or send to someone. You can do this easily on a computer, but it is harder to do on an iPhone or iPad. This Shortcut will give you a way to do it by sharing the photo to create a smaller file.